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We present a novel approach, which will potentially allow for low-temperature-substrate synthesis of carbon nanotubes using direct-current plasma-enhanced chemical vapour deposition. The approach utilizes top-down plasma heating rather than conventional heating from a conventional substrate heater under the electrode. In this work, a relatively thick titanium layer is used as a thermal barrier to create a temperature gradient between the Ni catalyst surface and the substrate. We describe the growth properties as a function of the bias voltage and the hydrocarbon concentrations. The heating during growth is provided solely by the plasma, which is dependent only on the process conditions, which dictate the power density and the cooling of the substrate, plus now the thermal properties of the "barrier layer". This novel approach of using plasma heating and thermal barrier allows for the synthesis of carbon nanotubes at low substrate temperature conditions to be attained with suitable cooling schemes.

The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/mum) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 mum in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/mum) for emission.

Pulse laser ablation and subsequent laser nanostructuring at room temperature has been employed to produce nanostructured Ni on SiO2/Si substrates for catalytic growth of carbon nanotubes. The resultant nanostructured surface is seen to consist of nanometer sized hemispherical droplets whose mean diameter is controlled by the initial metal thickness, which in turn is readily controlled by the number of laser pulses. Vertically aligned multiwall carbon nanotube mats were then grown using conventional plasma enhanced chemical vapor deposition. We show that within a single processing technique it is possible to produce the initial metal-on-oxide thin film to a chosen thickness but also to be able to alter the morphology of the film to desired specifications at low macroscopic temperatures using the laser parameters. The influence of the underlying oxide is also explored to explain the mechanism of nanostructuring of the Ni catalyst. (C) 2004 American Institute of Physics.

Traditionally, the emission of electrons from materials have been explained using either the Fowler–Nordheim emission mechanism where high electric fields are used to extract electrons from surfaces or using conventional thermal emission where high currents are used to “boil” off electrons to vacuum. In this letter, we propose an alternative mechanism for electron emission from highly compressive thin films based on stress-induced “band structure” modification of nano-ordered sp2 regions in the thin films. Experimental results are recorded which show that the localized compressive stress governs electron emission in the amorphous carbon thin films studied here rather than the surface nanostructures/features or the diamond-like sp3 hybridized bond component. This analysis is in agreement with the concept of an internal or nongeometric field enhancement from sp2 nanostructures giving rise to high dielectric inhomogeneity within the carbon thin film. The results presented could be extended to explain the ano...

In this work, Ag–SiO2 nanocomposite layers were synthesized by introducing Ag nanoclusters into thermally oxidized SiO2 layers, using ion implantation. The field-emission (FE) properties of these layers were studied and correlated with the results from atomic force microscopy and transmission electron microscopy measurements. These nanocomposites exhibit good FE properties and give an emission current of 1nA at electric fields as low as 13V∕μm, for a dose of 5×1016Ag+∕cm2, compared with 204V∕μm for “bare” SiO2 layers. It is clearly demonstrated that the good FE properties of these nanocomposites are attributed to two types of local-field enhancement: one due to the surface morphology and the other due to electrical inhomogeneity. The isolated conductive Ag nanoclusters embedded in the electrically insulating SiO2 matrix provide a field enhancement due to the electrical inhomogeneity effect. Moreover, the implanted Ag ions diffuse to the surface, during the implantation process, and create dense surface-pr...

Chemical vapor-synthesized carbon nanotubes are typically grown at temperatures around 600 °C. We report on the deployment of a titanium layer to help elevate the constraints on the substrate temperature during plasma-assisted growth. The growth is possible through the lowering of the hydrocarbon content used in the deposition, with the only source of heat provided by the plasma. The nanotubes synthesized have a small diameter distribution, which deviates from the usual trend that the diameter is determined by the thickness of the catalyst film. Simple thermodynamic simulations also show that the quantity of heat, that can be distributed, is determined by the thickness of the titanium layer. Despite the lower synthesis temperature, it is shown that this technique allows for high growth rates as well as better quality nanotubes. © 2005 American Institute of Physics.

Diamond-like carbon (DLC) films have always had high intrinsic stress due to their metastable structure and the fine balance between film density and bond stability. We show the effects of high intrinsic stress on the electron field emission performance, where a lower electric field for emission is recorded with increasing stress in the DLC films. In addition to examining “as deposited” films with different magnitudes of intrinsic stress, we subject the DLC films to external pressure by physically bending the a-C∕silicon substrates. The result is a phenomenon where electrons are “squeezed” out of the films, and can be applied to the fabrication of stress sensors.

Vertically aligned carbon nanofibers (VACNFs) were synthesized on Ni-coated Si substrates using a dc plasma-enhanced chemical-vapor deposition system. The size of the Ni islands used as catalyst to grow the VACNFs was formed by both thermal annealing and laser processing on thin metal layers. It was observed that the diameter of the carbon nanofibers is strongly dependent on the initial Ni island dimension. By varying the laser power from 228 to 279mJ∕cm2, the size of these Ni islands can be controlled independent of the initial Ni film thickness. Electron field-emission results show that the emission threshold field is dependent on both the height and radius of these VACNFs and also field shielding effects. Threshold fields as low as 2V∕μm was obtained from the sample with the largest height over radius ratio.